Pre-hospital care plays a crucial role in determining health outcomes after trauma and out-of-hospital complications of other leading causes of death, such as stroke and heart disease. Yet, it has largely been neglected by the scientific community, leaving assumptions rather than evidence to guide policy decisions on the ambulance type and hospital destination that would optimize a patient?s health outcomes. In previous work, our team compared two competing strategies for delivering emergency ambulance care, advanced life support (ALS) and basic life support (BLS), and found that ALS, which is much more common in the United States, may be less successful in keeping patients alive and as well as possible. In this work, we studied several out-of- hospital medical emergencies using quasi-experimental research methods and a large national sample of Medicare claims from 2006-2011. In our first aim of the proposed work, we will extend this analysis to the under 65 adult Medicaid population, which is particularly important to do for major trauma as younger people sustain different patterns of injury than older adults. In addition to major trauma, we will also study out-of-hospital cardiac arrest, stroke, respiratory failure, and acute myocardial infarction, all of which are time sensitive conditions requiring prompt medical care. The next aim will build upon this and our previous work and analyze choice of hospital destination for ALS and BLS patients. To date, regionalization studies that evaluate outcomes of out-of- hospital medical emergencies at specialty hospitals like trauma centers rarely account for the amount of time spent in pre-hospital care, which is influenced by whether that care was provided by ALS or BLS, or adequately study the trade-off between minimizing travel distance and optimizing hospital characteristics. We will improve upon the existing literature using Medicare and Medicaid claims, regression analysis, and causal inference methods to study the associations and causal relationships between these key factors and survival. To describe the broad implications of our findings, in our third aim we will quantify the potential benefits at the national level of optimizing individual pre-hospital care decisions based on existing resources using a microsimulation model. Finally, for longer-term policymaking, we will assess how shifts in ambulance resources and hospital locations in four states might affect population health outcomes for out-of-hospital medical emergencies. The proposed work will advance safety and quality research for the pre-hospital care system and hospital regionalization and will fill critical gaps in the scientific literature. We expect to provide specific, evidence-based insights that policymakers can use to substantially improve pre-hospital and hospital care for out-of-hospital medical emergencies. Importantly, our use of Medicaid claims and focus on trauma care will be particularly relevant to inner-city, low-income, and minority populations.